Illustrated herein, in various embodiments, is an assembly for use with ultraviolet curable inks wherein curing is performed using ultraviolet light emitting diodes (UV-LEDs). In particular, one or more UV-LEDs are placed in a geometry corresponding to one or more individual printhead ejectors upon the assembly such that when a printhead ejector deposits an ink droplet upon a substrate, at least one UV-LED will subsequently pass directly over the droplet. A process for printing utilizing such an assembly is also disclosed herein in various embodiments.
A relatively new printing technology exists that increases printing speed with fast controllable drying, ultraviolet (UV) photosensitive resin-containing substances. Fast drying substances containing ultraviolet photosensitive resins work well with direct marking print technology near room temperature. As used here, the term “ultraviolet” encompasses the range of wavelengths of light from about 50 nanometers to about 500 nanometers.
Ultraviolet photosensitive inks may be used in inkjet printers. Two main inkjet technologies are currently generally used. In a “bubble jet” or thermal inkjet (TIJ) printer, each printhead ejector comprises a reservoir, a heating element, and a nozzle. When the heating element heats up, some of the ink is vaporized to create a bubble within the reservoir. As the bubble expands, an ink droplet is pushed out of the nozzle. When the bubble collapses, a vacuum is created which pulls ink into the reservoir from the ink cartridge. TIJ printers typically use inks in a solvent (such as water) having a low viscosity of about 2 centipoises (cPs).
In a piezoelectric inkjet (PIJ) printer, each printhead ejector comprises a piezoelectric crystal at one end, a nozzle at the other end, and a reservoir between them. When an electric current is applied to the crystal, it vibrates. As the crystal vibrates inward (into the reservoir), an ink droplet is pushed out of the nozzle. When the crystal vibrates outward, a vacuum is created which pulls ink into the reservoir from the ink cartridge. The ink used in a PIJ printer typically has a viscosity of about 10 to 12 cPs. In both cases, the ink droplets form the image to be printed.
Another type of drop-on-demand system is known as acoustic ink printing (AIP). As is known, an acoustic beam exerts a radiation pressure against objects upon which it impinges. Thus, when an acoustic beam impinges on a free surface (i.e., liquid/air interface) of a pool of liquid from beneath, the radiation pressure which it exerts against the surface of the pool may reach a sufficiently high level to release individual droplets of liquid from the pool, despite the restraining force of surface tension.
Because a PIJ printer operates at a higher viscosity range, a solvent-free UV-curable ink formulation can be used. This means that there are no VOC (volatile organic compound) emissions; the lack of emissions and durability are the major attractive features of UV-curable inks. Such formulations are known to those skilled in the art and can be manufactured using photoinitiators and mixtures of curable monomers and oligomers. Suitable photoinitiators may be selected from a wide variety of compounds that respond to light through production of free radicals; alternatively photoinitiators may be selected from a variety of compounds that respond to light through production of Bronsted or Lewis acids. When free radical photoinitiators are employed the typical polymerizable groups on the monomer may be acrylates or methacrylates. When strong acid or cationic photoinitiators are used the typical polymerizable groups are epoxides and vinyl ethers.
In a printer using UV-curable inks, the UV light source has traditionally been a mercury vapor lamp. Recently, there has been a trend towards using UV light emitting diodes (UV-LEDs). UV-LEDs offer several advantages over mercury vapor lamps. They can be turned on and used instantly (“instant-on”), whereas medium pressure mercury vapor lamps typically require many minutes to stabilize before they can be used. Microwave excited or electrodeless mercury vapor lamps may require several seconds to switch on and off. UV LEDs also produce less heat, do not produce byproducts such as ozone, and have a longer operating life. In addition, they produce a narrow distribution of wavelengths (±10-15 nanometers), leading to a highly energy-efficient cure. Because the distribution of wavelengths is narrow, a UV-LED can also be used to selectively cure mixtures containing multiple photoinitiators (P is) which respond to different wavelengths. UV-LEDs are also smaller and potentially cheaper, which allow them to be placed and used in locations not previously suitable for a large mercury vapor lamp systems.
Additionally, mercury lamps and xenon light sources flood the curing target with light and are not digitally addressable. UV LEDs as configured in this disclosure on the other hand can be digitally addressed; the light source can be turned on and off to illuminate only the deposited ink pixel and not illuminate areas where no ink has been placed. If no ink drop has been fired at a particular time from a particular ink jet orifice, the corresponding LED(s) need not be fired. In this fashion significant energy savings may be realized, a document that has only 10% area coverage would need only 10% of the light that complete 100% coverage would require. This process is digital curing, it is readily accomplished as the digital code has already been created for controlling the printhead and can be extended to command the firing of the corresponding LEDs.
Furthermore, with direct marking print technologies, such as inkjet applications, drop diameter spread control directly impacts the quality of print image resolution. To minimize lateral ink spread, the drop diameter needs to be controlled and minimized, generally by using various ink delivery technologies. One method to minimize ink spread is to cure the ink as quickly as possible after delivery by increasing its viscosity. In printers where an intermediate transfer surface, such as a transfuse drum, is used before transferring a UV-curable ink to a final substrate such as paper, the ink may be partially cured on the intermediate transfer surface before it is transferred to the final substrate and cured again. For example, the ink may begin with a viscosity around 10 cPs within the ink cartridge; after being deposited onto the intermediate transfer surface, it is partially cured to a transfuse viscosity of between 104 and 109 cPs. This ensures a stable image during drum rotation and effective transfer to paper. After it is transferred to the final substrate, it is completely cured to an almost infinite viscosity.
In an inkjet printer which uses a transfuse drum, the image is usually built up on the drum over several rotations of the drum. If the transfuse drum rotates along the y-axis and the axis of the transfuse drum defines the x-axis, then the printhead deposits a set of ink drops onto the drum as the drum rotates along the y-axis. The printhead then moves along the x-axis to a new location along the drum where it deposits another set of ink drops during the next rotation. This rotate-and-translate scheme reduces the complexity and cost of the printhead by reducing the number of printhead ejectors required to print the image. Similarly, it reduces the cost and complexity of the LED array, fewer LED elements or dies are required and they can be more widely spaced. It also reduces the number of print defects in the final image; if one printhead ejector fails to deposit an ink drop, the failure can be detected and masked by other ejectors passing over the same spot. The number of rotations and corresponding printhead translations required to produce an image can vary; for example, it can range from 4 to 22 rotations.
As mentioned above, one method to minimize ink spread and provide a defect-free image is to cure the ink as quickly as possible after delivery by increasing its viscosity. For UV-curable inks, ideally this means placing the UV light source as close to the printhead ejector as possible; this reduces the time during which the low-viscosity ink can spread.
It is therefore desirable to provide an apparatus for inkjet printers which provides direct UV light energy for partially or completely curing UV-curable inks in a minimum amount of time after the ink has been deposited.